The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Uninterruptible power supplies (UPSs) provide power to critical equipment that cannot experience any break in service. In other words, a UPS is used in circumstances where even a short duration brownout or blackout is unacceptable. Examples of such critical equipment include computer servers, computer networks, telecommunication electronics, medical devices, security networks, and the like. An uninterruptable power supply makes regulated power available to the critical equipment regardless of the status of the power supply from the power grid.
FIG. 1 shows a schematic of a conventional uninterruptable power supply. The UPS system consists of main input switch 8, input filter circuit 10, rectifier circuit 14, main capacitor module 16, voltage boost circuit 18 and output inverter module 22. The input filter circuit 10 includes a current filter 7 and a filter capacitor 12.
To increase the redundancy of the power system, a generator 2 can also be connected to the UPS during power failures. Normally switching between commercial power 4 and generator 2 will be performed by a mechanical switch 6. During the time interval from when commercial power is lost, until switching to generator power, the UPS will supply power by using a series of batteries 20. Normally when running on battery power, the voltage supplied to power bus 15 is boosted up using voltage boost up circuit 18. The voltage boost up circuit 18 controls the DC bus voltage (Vdc) during battery backup mode, as described below.
When the UPS is in an AC line mode, either from the power grid or the generator, the power bus voltage Vdc is normally decided by commutation from the AC input power. For example, in the case of a 480V bus voltage, the corresponding AC power is 680V; (680V/√2=480V, nominal voltage).
When in the battery backup mode, the voltage boost circuit will raise up the DC power bus 15 voltage from the battery voltage. The bus voltage is usually raised to a value lower than nominal voltage due to economical reasons regarding battery usage. However, if the commercial power is out for an extended period of time, it is impractical to use only battery power as there is not enough energy stored in the batteries 20. Thus, the UPS will switch from battery power to generator power.
When switching from battery backup mode to AC line mode, the power bus 18 voltage will be boosted up to a level higher than the nominal level. Voltage boost circuit 18 will stop boosting after switching to AC mode.
However, a problem occurs with the conventional UPS. During the switching of power from the battery to AC line power, there is a high current flow due to the potential difference between the AC line's commutated voltage and the DC power bus 15 voltage. Especially in the case where a generator is connected to the AC input line and supplies AC power to the UPS, this current may become large due to the oscillation by the generator's internal impedance (inductance) and input filtering capacitor. This high current may cause instability in the output voltage of the generator. If this happens, the UPS may detect this unstable voltage and/or current, and move to battery backup mode again. The operation of the convention UPS will be explained in greater detail below with respect to FIG. 2
FIG. 2 is a timing diagram showing currents, voltages and switches according to the conventional UPS design as shown in FIG. 1. At time t<t1, switching element 8 is closed. That is, switching element 8 is conducting current, allowing current to flow from the commercial power grid 4 to the UPS 1.
As shown at time t1, commercial AC power is lost and battery power immediately takes over in powering the UPS 1. The voltage from batteries 20 is boosted by voltage boost circuit 18. Once commercial AC power is lost, generator 2 turns on and begins to power up. After a certain time from the time AC power is lost, and once generator 2 has come up to full power and has stabilized, at time t1<t<t2, input switch 6 switches from commercial AC power to generator 2 power.
Then at time t2, switching element 8 closes and allows current from the generator 2 to flow into UPS 1. Concurrently, battery power and voltage boost circuit 18 are switched off in order to get power from the generator 2. As shown in FIG. 2, the generator output voltage oscillates wildly, causing the bus voltage to oscillate as well. This oscillation is due to the resonance created when the high input voltage of the generator 2 is connected to the lower voltage of the power bus 15, powered by voltage boost circuit 18 and batteries 20.
That is, the inrush current caused by the sudden change from the battery boost circuit to generator power overwhelms the generator 2's power output capabilities. The generator 2 cannot supply such a large current at such a high voltage. As such, the voltage of the generator 2 has a sudden drop. This leads to an oscillatory effect which can damage the generator.
As shown in FIG. 2, the generator output voltage and output current oscillate wildly. Main switch 8 will open because the sudden drop in voltage from the generator 2, appears to the UPS control circuitry to be a power failure. Thus, as shown at time t4, the main switch opens and the bus is again powered by voltage boost circuit 18 and batteries 20.
Thus, in the case where an inrush current from the generator 2 to the UPS 1 is too great, the mode change between AC line mode and battery backup mode repeats and additional battery power is consumed.